Recently, some techniques have been developed to diagnose the state of a disease, such as cancer, in living tissue using a chemical agent that accumulates in a diseased area such as cancer and that emits fluorescence when irradiated with excitation light. In particular, a technique is known in which a living body is irradiated with excitation light from, for example, a fluorescence endoscope, with the above chemical agent injected into the living body, fluorescence emitted from the chemical accumulated in the diseased area is detected as a two-dimensional image with, for example, the fluorescence endoscope, and the diseased area is diagnosed from the detected fluorescence image.
In such a technique, if the diseased area is located farther away than a normal area, the intensity of the fluorescence emitted from the chemical agent in the diseased area is lower because of the lower intensity per unit area of the excitation light irradiating the chemical agent in the diseased area. For example, there is a case where the excitation light diverges in a conical shape, and the intensity per unit area of the excitation light is approximately inversely proportional to the square of the distance between the distal end of the endoscope and the diseased area.
Various techniques have been proposed that can distinguish between a diseased area and a normal area without being affected by the difference between the distance to the diseased area and the distance to the normal area (see, for example, Patent Documents 1 to 3).
Patent Documents 1 and 2 mentioned above both disclose techniques for distinguishing between a diseased area and a normal area in a subject based on a fluorescence image related to fluorescence emitted from the subject and a reference-light image related to reflected light reflected from the subject upon irradiation with reference light. Specifically, the diseased area and the normal area are distinguished by determining the ratio of the luminance value of a predetermined region in the fluorescence image to that of the corresponding region in the reference-light image to reduce the effect of the distance difference.
In such a technique, reflected light specularly reflected from the surface of the subject can be detected, depending on the relative positional relationship between, for example, the distal end of the endoscope from which the reference light exits and the subject. That is, if reflected light of reference light specularly reflected from the surface of the subject is detected, only the luminance of the region where the specular reflection has occurred is prominent irrespective of the distance from the subject. Consequently, if the ratio of the luminance value of the fluorescence image to the luminance value of the reference light image is determined, the value of the ratio is significantly low only in the region where the specular reflection has occurred.
Patent Document 3 discloses a technique in which fluorescence in two wavelength bands emitted from a subject by irradiation with excitation light is detected, and a diseased area and a normal area in the subject are distinguished based on the ratio of the intensity value of the fluorescence in one wavelength band to the intensity value of the fluorescence in the other wavelength band. In this case, only the fluorescence emitted from the subject is detected, and no light reflected from the surface of the subject is detected, so that the diseased area and the normal area can be distinguished irrespective of whether or not specular reflection has occurred at the surface of the subject.
Patent Document 1:    Japanese Unexamined Patent Application, Publication No. 2001-137173
Patent Document 2:    Japanese Unexamined Patent Application, Publication No. 2003-290130
Patent Document 3:    Japanese Unexamined Patent Application, Publication No. 2004-024932